Index: Makefile.inc1
===================================================================
--- Makefile.inc1
+++ Makefile.inc1
@@ -2342,6 +2342,8 @@
 .endif
 # mkfifo is used by sys/conf/newvers.sh
 _basic_bootstrap_tools+=usr.bin/mkfifo
+# jot is needed for the mkimg tests
+_basic_bootstrap_tools+=usr.bin/jot
 
 .if ${MK_BOOT} != "no"
 # md5 is used by boot/beri (and possibly others)
Index: usr.bin/mkimg/tests/Makefile
===================================================================
--- usr.bin/mkimg/tests/Makefile
+++ usr.bin/mkimg/tests/Makefile
@@ -15,9 +15,25 @@
 	sed -e '/^#.*/D' < ${.ALLSRC} > ${.TARGET}
 .endfor
 
+# Note: Pre-generating this test file saves a lot of time when building on
+# emulated platforms such as QEMU. It can take about 2-5 seconds to generate
+# the test file using jot (depending on the emulated architecture) and this
+# is done for each of the 168 test configurations.
+# The effect is even more pronounced on CHERI-RISCV QEMU (emulating CHERI inside
+# QEMU adds additional run-time overhead): Running the apm_1x1_512_raw without
+# the pre-generated file takes about 108 seconds of which 102 seconds (over 95%)
+# were spent running jot -b. It's even worse on CHERI-MIPS QEMU: 187 seconds
+# for jot -b P 2097152 > /dev/null. By using a pre-generated 4MB file, the
+# slowest test variant (vtoc8_63x255_4096_vhdx) now only takes 29 seconds (of
+# which 26s are spent in hexdump -C) instead of previously 2min30s.
+${PACKAGE}FILES+=	partition_data_4M.bin
+partition_data_4M.bin: Makefile
+	jot -b P 2097152 > ${.TARGET} || rm -f ${.TARGET}
+
 CLEANFILES+=	${${PACKAGE}FILES}}
 
-rebase: .PHONY
-	(cd ${.CURDIR}; /usr/libexec/atf-sh ${_REBASE_SCRIPT}.sh rebase)
+rebase: partition_data_4M.bin ${_REBASE_SCRIPT} .PHONY
+	cd ${.CURDIR}; PATH=${.OBJDIR}/..:$${PATH}:/usr/bin:/bin \
+	    /usr/libexec/atf-sh ${.OBJDIR}/${_REBASE_SCRIPT} -s ${.OBJDIR} rebase
 
 .include <bsd.test.mk>
Index: usr.bin/mkimg/tests/mkimg_test.sh
===================================================================
--- usr.bin/mkimg/tests/mkimg_test.sh
+++ usr.bin/mkimg/tests/mkimg_test.sh
@@ -56,7 +56,7 @@
     if test -z "$partarg"; then
 	local swap ufs
 	swap="-p freebsd-swap::128K"
-	ufs="-p freebsd-ufs:=`mkcontents P 4194304`"
+	ufs="-p freebsd-ufs:=$(atf_get_srcdir)/partition_data_4M.bin"
 	partarg="$ufs $swap"
     fi